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PERLFAQ7(1) Perl Programmers Reference Guide PERLFAQ7(1)
NAME
perlfaq7 - General Perl Language Issues ($Revision: 1.28 $, $Date: 2005/12/31
00:54:37 $)
DESCRIPTION
This section deals with general Perl language issues that don't clearly fit into
any of the other sections.
Can I get a BNF/yacc/RE for the Perl language?
There is no BNF, but you can paw your way through the yacc grammar in perly.y in
the source distribution if you're particularly brave. The grammar relies on very
smart tokenizing code, so be prepared to venture into toke.c as well.
In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF. The work
of parsing perl is distributed between yacc, the lexer, smoke and mirrors."
What are all these $@%&* punctuation signs, and how do I know when to use them?
They are type specifiers, as detailed in perldata:
$ for scalar values (number, string or reference)
@ for arrays
% for hashes (associative arrays)
& for subroutines (aka functions, procedures, methods)
* for all types of that symbol name. In version 4 you used them like
pointers, but in modern perls you can just use references.
There are couple of other symbols that you're likely to encounter that aren't
really type specifiers:
<> are used for inputting a record from a filehandle.
\ takes a reference to something.
Note that <FILE> is neither the type specifier for files nor the name of the han-
dle. It is the "<>" operator applied to the handle FILE. It reads one line (well,
record--see "$/" in perlvar) from the handle FILE in scalar context, or all lines
in list context. When performing open, close, or any other operation besides "<>"
on files, or even when talking about the handle, do not use the brackets. These
are correct: "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN to FILE".
Do I always/never have to quote my strings or use semicolons and commas?
Normally, a bareword doesn't need to be quoted, but in most cases probably should
be (and must be under "use strict"). But a hash key consisting of a simple word
(that isn't the name of a defined subroutine) and the left-hand operand to the "=>"
operator both count as though they were quoted:
This is like this
------------ ---------------
$foo{line} $foo{'line'}
bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a list. Good
style (see perlstyle) says to put them in except for one-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
@lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
How do I skip some return values?
One way is to treat the return values as a list and index into it:
$dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements that you need:
($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the "use warnings" pragma allows fine con-
trol of what warning are produced. See perllexwarn for more details.
{
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
Additionally, you can enable and disable categories of warnings. You turn off the
categories you want to ignore and you can still get other categories of warnings.
See perllexwarn for the complete details, including the category names and hierar-
chy.
{
no warnings 'uninitialized';
$a = $b + $c;
}
If you have an older version of Perl, the $^W variable (documented in perlvar) con-
trols runtime warnings for a block:
{
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
Note that like all the punctuation variables, you cannot currently use my() on $^W,
only local().
What's an extension?
An extension is a way of calling compiled C code from Perl. Reading perlxstut is a
good place to learn more about extensions.
Why do Perl operators have different precedence than C operators?
Actually, they don't. All C operators that Perl copies have the same precedence in
Perl as they do in C. The problem is with operators that C doesn't have, espe-
cially functions that give a list context to everything on their right, eg. print,
chmod, exec, and so on. Such functions are called "list operators" and appear as
such in the precedence table in perlop.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the super low prece-
dence "or" operator:
(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "not") deliberately have prece-
dence lower than that of list operators for just such situations as the one above.
Another operator with surprising precedence is exponentiation. It binds more
tightly even than unary minus, making "-2**2" product a negative not a positive
four. It is also right-associating, meaning that "2**3**2" is two raised to the
ninth power, not eight squared.
Although it has the same precedence as in C, Perl's "?:" operator produces an
lvalue. This assigns $x to either $a or $b, depending on the trueness of $maybe:
($maybe ? $a : $b) = $x;
How do I declare/create a structure?
In general, you don't "declare" a structure. Just use a (probably anonymous) hash
reference. See perlref and perldsc for details. Here's an example:
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try perltoot.
How do I create a module?
(contributed by brian d foy)
perlmod, perlmodlib, perlmodstyle explain modules in all the gory details. perlnew-
mod gives a brief overview of the process along with a couple of suggestions about
style.
If you need to include C code or C library interfaces in your module, you'll need
h2xs. h2xs will create the module distribution structure and the initial interface
files you'll need. perlxs and perlxstut explain the details.
If you don't need to use C code, other tools such as ExtUtils::ModuleMaker and Mod-
ule::Starter, can help you create a skeleton module distribution.
You may also want to see Sam Tregar's "Writing Perl Modules for CPAN" (
http://apress.com/book/bookDisplay.html?bID=14 ) which is the best hands-on guide
to creating module distributions.
How do I create a class?
See perltoot for an introduction to classes and objects, as well as perlobj and
perlbot.
How can I tell if a variable is tainted?
You can use the tainted() function of the Scalar::Util module, available from CPAN
(or included with Perl since release 5.8.0). See also "Laundering and Detecting
Tainted Data" in perlsec.
What's a closure?
Closures are documented in perlref.
Closure is a computer science term with a precise but hard-to-explain meaning. Clo-
sures are implemented in Perl as anonymous subroutines with lasting references to
lexical variables outside their own scopes. These lexicals magically refer to the
variables that were around when the subroutine was defined (deep binding).
Closures make sense in any programming language where you can have the return value
of a function be itself a function, as you can in Perl. Note that some languages
provide anonymous functions but are not capable of providing proper closures: the
Python language, for example. For more information on closures, check out any
textbook on functional programming. Scheme is a language that not only supports
but encourages closures.
Here's a classic function-generating function:
sub add_function_generator {
return sub { shift() + shift() };
}
$add_sub = add_function_generator();
$sum = $add_sub->(4,5); # $sum is 9 now.
The closure works as a function template with some customization slots left out to
be filled later. The anonymous subroutine returned by add_function_generator()
isn't technically a closure because it refers to no lexicals outside its own scope.
Contrast this with the following make_adder() function, in which the returned
anonymous function contains a reference to a lexical variable outside the scope of
that function itself. Such a reference requires that Perl return a proper closure,
thus locking in for all time the value that the lexical had when the function was
created.
sub make_adder {
my $addpiece = shift;
return sub { shift() + $addpiece };
}
$f1 = make_adder(20);
$f2 = make_adder(555);
Now "&$f1($n)" is always 20 plus whatever $n you pass in, whereas "&$f2($n)" is
always 555 plus whatever $n you pass in. The $addpiece in the closure sticks
around.
Closures are often used for less esoteric purposes. For example, when you want to
pass in a bit of code into a function:
my $line;
timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string, '$line = <STDIN>', there
would have been no way for the hypothetical timeout() function to access the lexi-
cal variable $line back in its caller's scope.
What is variable suicide and how can I prevent it?
This problem was fixed in perl 5.004_05, so preventing it means upgrading your ver-
sion of perl. ;)
Variable suicide is when you (temporarily or permanently) lose the value of a vari-
able. It is caused by scoping through my() and local() interacting with either
closures or aliased foreach() iterator variables and subroutine arguments. It used
to be easy to inadvertently lose a variable's value this way, but now it's much
harder. Take this code:
my $f = 'foo';
sub T {
while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" }
}
T;
print "Finally $f\n";
If you are experiencing variable suicide, that "my $f" in the subroutine doesn't
pick up a fresh copy of the $f whose value is <foo>. The output shows that inside
the subroutine the value of $f leaks through when it shouldn't, as in this output:
foobar
foobarbar
foobarbarbar
Finally foo
The $f that has "bar" added to it three times should be a new $f "my $f" should
create a new lexical variable each time through the loop. The expected output is:
foobar
foobar
foobar
Finally foo
How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
With the exception of regexes, you need to pass references to these objects. See
"Pass by Reference" in perlsub for this particular question, and perlref for infor-
mation on references.
See "Passing Regexes", below, for information on passing regular expressions.
Passing Variables and Functions
Regular variables and functions are quite easy to pass: just pass in a refer-
ence to an existing or anonymous variable or function:
func( \$some_scalar );
func( \@some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
Passing Filehandles
As of Perl 5.6, you can represent filehandles with scalar variables which you
treat as any other scalar.
open my $fh, $filename or die "Cannot open $filename! $!";
func( $fh );
sub func {
my $passed_fh = shift;
my $line = <$fh>;
}
Before Perl 5.6, you had to use the *FH or "\*FH" notations. These are "type-
globs"--see "Typeglobs and Filehandles" in perldata and especially "Pass by
Reference" in perlsub for more information.
Passing Regexes
To pass regexes around, you'll need to be using a release of Perl sufficiently
recent as to support the "qr//" construct, pass around strings and use an
exception-trapping eval, or else be very, very clever.
Here's an example of how to pass in a string to be regex compared using "qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);
Notice how "qr//" allows flags at the end. That pattern was compiled at com-
pile time, although it was executed later. The nifty "qr//" notation wasn't
introduced until the 5.005 release. Before that, you had to approach this
problem much less intuitively. For example, here it is again if you don't have
"qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
}
$match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
return eval "\$val =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due to the double interpola-
tion of the eval and the double-quoted string. For example:
$pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
eval "\$string =~ /$pattern_of_evil/";
Those preferring to be very, very clever might see the O'Reilly book, Mastering
Regular Expressions, by Jeffrey Friedl. Page 273's Build_MatchMany_Function()
is particularly interesting. A complete citation of this book is given in
perlfaq2.
Passing Methods
To pass an object method into a subroutine, you can do this:
call_a_lot(10, $some_obj, "methname")
sub call_a_lot {
my ($count, $widget, $trick) = @_;
for (my $i = 0; $i < $count; $i++) {
$widget->$trick();
}
}
Or, you can use a closure to bundle up the object, its method call, and argu-
ments:
my $whatnot = sub { $some_obj->obfuscate(@args) };
func($whatnot);
sub func {
my $code = shift;
&$code();
}
You could also investigate the can() method in the UNIVERSAL class (part of the
standard perl distribution).
How do I create a static variable?
(contributed by brian d foy)
Perl doesn't have "static" variables, which can only be accessed from the function
in which they are declared. You can get the same effect with lexical variables,
though.
You can fake a static variable by using a lexical variable which goes out of scope.
In this example, you define the subroutine "counter", and it uses the lexical vari-
able $count. Since you wrap this in a BEGIN block, $count is defined at com-
pile-time, but also goes out of scope at the end of the BEGIN block. The BEGIN
block also ensures that the subroutine and the value it uses is defined at compile-
time so the subroutine is ready to use just like any other subroutine, and you can
put this code in the same place as other subroutines in the program text (i.e. at
the end of the code, typically). The subroutine "counter" still has a reference to
the data, and is the only way you can access the value (and each time you do, you
increment the value). The data in chunk of memory defined by $count is private to
"counter".
BEGIN {
my $count = 1;
sub counter { $count++ }
}
my $start = count();
.... # code that calls count();
my $end = count();
In the previous example, you created a function-private variable because only one
function remembered its reference. You could define multiple functions while the
variable is in scope, and each function can share the "private" variable. It's not
really "static" because you can access it outside the function while the lexical
variable is in scope, and even create references to it. In this example, "incre-
ment_count" and "return_count" share the variable. One function adds to the value
and the other simply returns the value. They can both access $count, and since it
has gone out of scope, there is no other way to access it.
BEGIN {
my $count = 1;
sub increment_count { $count++ }
sub return_count { $count }
}
To declare a file-private variable, you still use a lexical variable. A file is
also a scope, so a lexical variable defined in the file cannot be seen from any
other file.
See "Persistent Private Variables" in perlsub for more information. The discussion
of closures in perlref may help you even though we did not use anonymous subrou-
tines in this answer. See "Persistent Private Variables" in perlsub for details.
What's the difference between dynamic and lexical (static) scoping? Between
local() and my()?
"local($x)" saves away the old value of the global variable $x and assigns a new
value for the duration of the subroutine which is visible in other functions called
from that subroutine. This is done at run-time, so is called dynamic scoping.
local() always affects global variables, also called package variables or dynamic
variables.
"my($x)" creates a new variable that is only visible in the current subroutine.
This is done at compile-time, so it is called lexical or static scoping. my()
always affects private variables, also called lexical variables or (improperly)
static(ly scoped) variables.
For instance:
sub visible {
print "var has value $var\n";
}
sub dynamic {
local $var = 'local'; # new temporary value for the still-global
visible(); # variable called $var
}
sub lexical {
my $var = 'private'; # new private variable, $var
visible(); # (invisible outside of sub scope)
}
$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value "private" get printed. That's because $var
only has that value within the block of the lexical() function, and it is hidden
from called subroutine.
In summary, local() doesn't make what you think of as private, local variables. It
gives a global variable a temporary value. my() is what you're looking for if you
want private variables.
See "Private Variables via my()" in perlsub and "Temporary Values via local()" in
perlsub for excruciating details.
How can I access a dynamic variable while a similarly named lexical is in scope?
If you know your package, you can just mention it explicitly, as in
$Some_Pack::var. Note that the notation $::var is not the dynamic $var in the cur-
rent package, but rather the one in the "main" package, as though you had written
$main::var.
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
print "global is $main::var\n";
Alternatively you can use the compiler directive our() to bring a dynamic variable
into the current lexical scope.
require 5.006; # our() did not exist before 5.6
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
{
our $var;
print "global is $var\n";
}
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous subroutines are the same
ones that were in scope when the subroutine was created. In shallow binding, they
are whichever variables with the same names happen to be in scope when the subrou-
tine is called. Perl always uses deep binding of lexical variables (i.e., those
created with my()). However, dynamic variables (aka global, local, or package
variables) are effectively shallowly bound. Consider this just one more reason not
to use them. See the answer to "What's a closure?".
Why doesn't "my($foo) = <FILE>;" work right?
"my()" and "local()" give list context to the right hand side of "=". The <FH>
read operation, like so many of Perl's functions and operators, can tell which con-
text it was called in and behaves appropriately. In general, the scalar() function
can help. This function does nothing to the data itself (contrary to popular myth)
but rather tells its argument to behave in whatever its scalar fashion is. If that
function doesn't have a defined scalar behavior, this of course doesn't help you
(such as with sort()).
To enforce scalar context in this particular case, however, you need merely omit
the parentheses:
local($foo) = <FILE>; # WRONG
local($foo) = scalar(<FILE>); # ok
local $foo = <FILE>; # right
You should probably be using lexical variables anyway, although the issue is the
same here:
my($foo) = <FILE>; # WRONG
my $foo = <FILE>; # right
How do I redefine a builtin function, operator, or method?
Why do you want to do that? :-)
If you want to override a predefined function, such as open(), then you'll have to
import the new definition from a different module. See "Overriding Built-in Func-
tions" in perlsub. There's also an example in "Class::Template" in perltoot.
If you want to overload a Perl operator, such as "+" or "**", then you'll want to
use the "use overload" pragma, documented in overload.
If you're talking about obscuring method calls in parent classes, see "Overridden
Methods" in perltoot.
What's the difference between calling a function as &foo and foo()?
When you call a function as &foo, you allow that function access to your current @_
values, and you bypass prototypes. The function doesn't get an empty @_--it gets
yours! While not strictly speaking a bug (it's documented that way in perlsub), it
would be hard to consider this a feature in most cases.
When you call your function as "&foo()", then you do get a new @_, but prototyping
is still circumvented.
Normally, you want to call a function using "foo()". You may only omit the paren-
theses if the function is already known to the compiler because it already saw the
definition ("use" but not "require"), or via a forward reference or "use subs" dec-
laration. Even in this case, you get a clean @_ without any of the old values
leaking through where they don't belong.
How do I create a switch or case statement?
This is explained in more depth in the perlsyn. Briefly, there's no official case
statement, because of the variety of tests possible in Perl (numeric comparison,
string comparison, glob comparison, regex matching, overloaded comparisons, ...).
Larry couldn't decide how best to do this, so he left it out, even though it's been
on the wish list since perl1.
Starting from Perl 5.8 to get switch and case one can use the Switch extension and
say:
use Switch;
after which one has switch and case. It is not as fast as it could be because it's
not really part of the language (it's done using source filters) but it is avail-
able, and it's very flexible.
But if one wants to use pure Perl, the general answer is to write a construct like
this:
for ($variable_to_test) {
if (/pat1/) { } # do something
elsif (/pat2/) { } # do something else
elsif (/pat3/) { } # do something else
else { } # default
}
Here's a simple example of a switch based on pattern matching, this time lined up
in a way to make it look more like a switch statement. We'll do a multiway condi-
tional based on the type of reference stored in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};
# DEFAULT
warn "User defined type skipped";
}
See "perlsyn/"Basic BLOCKs and Switch Statements"" for many other examples in this
style.
Sometimes you should change the positions of the constant and the variable. For
example, let's say you wanted to test which of many answers you were given, but in
a case-insensitive way that also allows abbreviations. You can use the following
technique if the strings all start with different characters or if you want to
arrange the matches so that one takes precedence over another, as "SEND" has prece-
dence over "STOP" here:
chomp($answer = <>);
if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of function references.
my %commands = (
"happy" => \&joy,
"sad", => \&sullen,
"done" => sub { die "See ya!" },
"mad" => \&angry,
);
print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
$commands{$string}->();
} else {
print "No such command: $string\n";
}
How can I catch accesses to undefined variables, functions, or methods?
The AUTOLOAD method, discussed in "Autoloading" in perlsub and "AUTOLOAD: Proxy
Methods" in perltoot, lets you capture calls to undefined functions and methods.
When it comes to undefined variables that would trigger a warning under "use warn-
ings", you can promote the warning to an error.
use warnings FATAL => qw(uninitialized);
Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused, you've misspelled the
method name, or the object is of the wrong type. Check out perltoot for details
about any of the above cases. You may also use "print ref($object)" to find out
the class $object was blessed into.
Another possible reason for problems is because you've used the indirect object
syntax (eg, "find Guru "Samy"") on a class name before Perl has seen that such a
package exists. It's wisest to make sure your packages are all defined before you
start using them, which will be taken care of if you use the "use" statement
instead of "require". If not, make sure to use arrow notation (eg.,
"Guru->find("Samy")") instead. Object notation is explained in perlobj.
Make sure to read about creating modules in perlmod and the perils of indirect
objects in "Method Invocation" in perlobj.
How can I find out my current package?
If you're just a random program, you can do this to find out what the currently
compiled package is:
my $packname = __PACKAGE__;
But, if you're a method and you want to print an error message that includes the
kind of object you were called on (which is not necessarily the same as the one in
which you were compiled):
sub amethod {
my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
}
How can I comment out a large block of perl code?
You can use embedded POD to discard it. Enclose the blocks you want to comment out
in POD markers. The <=begin> directive marks a section for a specific formatter.
Use the "comment" format, which no formatter should claim to understand (by pol-
icy). Mark the end of the block with <=end>.
# program is here
=begin comment
all of this stuff
here will be ignored
by everyone
=end comment
=cut
# program continues
The pod directives cannot go just anywhere. You must put a pod directive where the
parser is expecting a new statement, not just in the middle of an expression or
some other arbitrary grammar production.
See perlpod for more details.
How do I clear a package?
Use this code, provided by Mark-Jason Dominus:
sub scrub_package {
no strict 'refs';
my $pack = shift;
die "Shouldn't delete main package"
if $pack eq "" || $pack eq "main";
my $stash = *{$pack . '::'}{HASH};
my $name;
foreach $name (keys %$stash) {
my $fullname = $pack . '::' . $name;
# Get rid of everything with that name.
undef $$fullname;
undef @$fullname;
undef %$fullname;
undef &$fullname;
undef *$fullname;
}
}
Or, if you're using a recent release of Perl, you can just use the Sym-
bol::delete_package() function instead.
How can I use a variable as a variable name?
Beginners often think they want to have a variable contain the name of a variable.
$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works sometimes, but it is a very bad idea for two reasons.
The first reason is that this technique only works on global variables. That means
that if $fred is a lexical variable created with my() in the above example, the
code wouldn't work at all: you'd accidentally access the global and skip right over
the private lexical altogether. Global variables are bad because they can easily
collide accidentally and in general make for non-scalable and confusing code.
Symbolic references are forbidden under the "use strict" pragma. They are not true
references and consequently are not reference counted or garbage collected.
The other reason why using a variable to hold the name of another variable is a bad
idea is that the question often stems from a lack of understanding of Perl data
structures, particularly hashes. By using symbolic references, you are just using
the package's symbol-table hash (like %main::) instead of a user-defined hash. The
solution is to use your own hash or a real reference instead.
$USER_VARS{"fred"} = 23;
$varname = "fred";
$USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic references. Sometimes
this comes up in reading strings from the user with variable references and wanting
to expand them to the values of your perl program's variables. This is also a bad
idea because it conflates the program-addressable namespace and the user-address-
able one. Instead of reading a string and expanding it to the actual contents of
your program's own variables:
$str = 'this has a $fred and $barney in it';
$str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS and have variable refer-
ences actually refer to entries in that hash:
$str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous approach. Of course, you don't
need to use a dollar sign. You could use your own scheme to make it less confus-
ing, like bracketed percent symbols, etc.
$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a variable to contain the name
of a variable is because they don't know how to build proper data structures using
hashes. For example, let's say they wanted two hashes in their program: %fred and
%barney, and that they wanted to use another scalar variable to refer to those by
name.
$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
$name = "barney";
$$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled with the problems enumer-
ated above. It would be far better to write:
$folks{"fred"}{WIFE} = "wilma";
$folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
The only times that you absolutely must use symbolic references are when you really
must refer to the symbol table. This may be because it's something that can't take
a real reference to, such as a format name. Doing so may also be important for
method calls, since these always go through the symbol table for resolution.
In those cases, you would turn off "strict 'refs'" temporarily so you can play
around with the symbol table. For example:
@colors = qw(red blue green yellow orange purple violet);
for my $name (@colors) {
no strict 'refs'; # renege for the block
*$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
}
All those functions (red(), blue(), green(), etc.) appear to be separate, but the
real code in the closure actually was compiled only once.
So, sometimes you might want to use symbolic references to directly manipulate the
symbol table. This doesn't matter for formats, handles, and subroutines, because
they are always global--you can't use my() on them. For scalars, arrays, and
hashes, though--and usually for subroutines-- you probably only want to use hard
references.
What does "bad interpreter" mean?
(contributed by brian d foy)
The "bad interpreter" message comes from the shell, not perl. The actual message
may vary depending on your platform, shell, and locale settings.
If you see "bad interpreter - no such file or directory", the first line in your
perl script (the "shebang" line) does not contain the right path to perl (or any
other program capable of running scripts). Sometimes this happens when you move
the script from one machine to another and each machine has a different path to
perl---/usr/bin/perl versus /usr/local/bin/perl for instance. It may also indicate
that the source machine has CRLF line terminators and the destination machine has
LF only: the shell tries to find /usr/bin/perl<CR>, but can't.
If you see "bad interpreter: Permission denied", you need to make your script exe-
cutable.
In either case, you should still be able to run the scripts with perl explicitly:
% perl script.pl
If you get a message like "perl: command not found", perl is not in your PATH,
which might also mean that the location of perl is not where you expect it so you
need to adjust your shebang line.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and other authors as
noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it under the same
terms as Perl itself.
Irrespective of its distribution, all code examples in this file are hereby placed
into the public domain. You are permitted and encouraged to use this code in your
own programs for fun or for profit as you see fit. A simple comment in the code
giving credit would be courteous but is not required.
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